1. Field of the Invention
This invention relates to a method of and a system for treatment of wastewater.
Most systems presently used for the purification of wastewater, such as industrial effluent or municipal sewage, employ methods based on biological agents. A broad class of systems bring aerobic bacteria and oxygen into close contact with the impurities within the wastewater. The bacteria can utilise at least some of these as nutrients for respiration and growth. There are two main types of system, plus derivatives thereof, currently in widespread use throughout the World.
The first main type of system is known as the ‘Activated Sludge’ system. Bacteria are held within particulate suspension of flocs known as sludge that are mixed with incoming wastewater. Air or oxygen is dispersed into this continuously flowing mixture, which is retained within one or more aeration tanks for a period of time sufficient for the bacteria to absorb or oxidise the nutrients within the wastewater. The flocs are then concentrated within a gravity settlement tank. A clear, purified stream leaves the top of this tank and the concentrated flocs (activated sludge) are recycled to the front of the system to be mixed with incoming wastewater. The retention time and degree of recycling of the sludge are such as to allow a specialised community of bacteria to develop which forms the activated sludge.
The second main type of system is known as the ‘Trickling Filter’ or ‘Biological Filter’ system. Here the wastewater is spread over the surface of a bed of media through which it then ‘trickles’ or percolates downwards and laterally. The media consists of individual elements or surfaces of sufficient size and so arranged that air is a continuous phase which is drawn into the bed by natural convection. A film of bacteria grows on the surfaces of the media under the liquid film and here purification takes place with the layer of bacteria and higher life-forms, known as a ‘biofilm’, growing thicker until the force of the down-flowing wastewater disrupts part of the layer and sweeps it out of the bed into a gravity settlement tank. The flow from the top of this tank is purified wastewater whilst the sheared-off film settles and accumulates in the bottom of the tank as sludge from where it is periodically drawn off. The distinction from the above-described activated sludge system is that the biofilm is not recycled but remains attached to the media until the shearing action of the trickling liquid dislodges it. Also air is introduced by natural convection, rather than forced convection as in the activated sludge system.
Developments and derivatives of these two main types of system are used which enable better effluent quality and/or lower operating costs and/or intensified smaller units to be achieved.
Two hybrid derivatives used are known as the “Biological Aerated Flooded Filter” (BAFF) system and the Submerged Aerated Filter (SAF) system.
The BAFF system uses fine media of granules of plastics or inorganic material upon which bacteria grow. Air is supplied to the bed directly but no settlement stage is required since the media is small enough to retain any solids, sheared from the bacterial film or included in the wastewater, by physical filtration and absorption. Eventually the pressure loss of flow of wastewater through the bed becomes too high, owing to the level of retained solids blocking the spaces among the media elements, and a cleaning system is required to remove solids accumulated in the bed. In the case of a system known as Biobead™ (United Kingdom Trade Mark Registration 1429397) and disclosed in EP-B-0533754, an increased rate of flow of air is used to put the media into motion, so resulting in cleaning of the media, in particular disruption of the biofilm, by shearing and attrition.
The SAF system uses larger media composed of large particles (in the form of irregularly shaped solid lumps or of plastics rings) or arrays of fixed surfaces. Such large particles may be of sizes of between, say, 20 mm. and 60 mm. Again air is introduced under pressure below the media bed for the purpose of aeration for aerobic biological treatment. Minimal solids filtration occurs because the size of the media is too large. Instead solids are eroded from the biofilm by the shearing action of influent flows and are then swept out of the media bed and onwards into a gravity settlement tank where solids separate and purified wastewater passes out at the top. Thus a media element cleaning system is not required during normal operation, which further distinguishes the SAF system from the BAFF system.
An essential difference between a BAFF system (as in WO91/18658) and the SAF system disclosed herein is that the BAFF system, mainly by virtue of having smaller media (the preferred maximum dimension disclosed in WO91/18658 being 3 mm. to 5 mm.), performs plural forms of treatment in a single stage, e.g. biological and solids removal. If a further stage is added, it performs further plural treatment in that stage. The SAF system, mainly owing to its larger media and fully mixed hydraulics (as opposed to plug flow hydraulics), can, when aerated, perform only a single form of biological treatment in a single stage and, in an aerobic biological treatment, is unable to retain solids; indeed solids are added to the effluent.
Even though SAF systems are supposedly self-cleaning, over a period of time the media bed can become blocked owing either to overload or to the accumulation of inert material such as fibres or plastic detritus. This risk also dictates that the media elements must be large and open-structured to minimize blockage. Smaller media elements would be more effective in the short term since the resulting higher surface area per unit volume and smaller niches among media elements would allow more bacteria and higher life-forms to be retained within the biofilm, which would increase the purification rate. In the longer operating timescale, owing to the blockage problem small-sized media elements are not practical.
The use of gravity settlement also limits performance, since small-sized effluent solids can escape with the final effluent flowing from the settlement tank, so reducing effluent quality. Also the process of de-nitrification of nitrified effluents can lift flocs of bacteria settled on the base of the tank by the growth and attachment of bubbles of nitrogen produced by the de-nitrification process. Further the large size of settlement tanks required by the gravity settlement process prevents their use where space is at premium.